Programming with OpenGL Part 0: 3D API March 1, 2007

2 Elements of Image Formation Objects Viewer Light source(s) Attributes that govern how light interacts with the materials in the scene Note the independence of the objects, viewer, and light source(s)

3 Pinhole Camera x p = -x/z/dy p = -y/z/d Use trigonometry to find projection of a point These are equations of simple perspective z p = d

4 Synthetic Camera Model center of projection image plane projector p projection of p

5 Advantages Separation of objects, viewer, light sources Two-dimensional graphics is a special case of three-dimensional graphics Leads to simple software API –Specify objects, lights, camera, attributes –Let implementation determine image Leads to fast hardware implementation

Programming with OpenGL Part 1: Background March 5, 2007

7 Objectives Development of the OpenGL API OpenGL Architecture –OpenGL as a state machine Functions –Types –Formats Simple program

8 Early History of APIs IFIPS (1973) formed two committees to come up with a standard graphics API –Graphical Kernel System (GKS) 2D but contained good workstation model –Core Both 2D and 3D –GKS adopted as IS0 and later ANSI standard (1980s) GKS not easily extended to 3D (GKS-3D) Far behind hardware development

9 PHIGS and X Programmers Hierarchical Graphics System (PHIGS) –Arose from CAD community –Database model with retained graphics (structures) X Window System –DEC/MIT effort –Client-server architecture with graphics PEX combined the two –Not easy to use (all the defects of each)

10 SGI and GL Silicon Graphics (SGI) revolutionized the graphics workstation by implementing the pipeline in hardware (1982) To use the system, application programmers used a library called GL With GL, it was relatively simple to program three dimensional interactive applications

11 OpenGL The success of GL lead to OpenGL (1992), a platform-independent API that was –Easy to use –Close enough to the hardware to get excellent performance –Focus on rendering –Omitted windowing and input to avoid window system dependencies

12 OpenGL Evolution Controlled by an Architectural Review Board (ARB) –Members include SGI, Microsoft, Nvidia, HP, 3DLabs,IBM,……. –Relatively stable (present version 1.4) Evolution reflects new hardware capabilities –3D texture mapping and texture objects –Vertex programs –Allows for platform specific features through extensions

13 OpenGL Libraries OpenGL core library –OpenGL32 on Windows –GL on most unix/linux systems OpenGL Utility Library (GLU) –Provides functionality in OpenGL core but avoids having to rewrite code Links with window system –GLX for X window systems –WGL for Widows –AGL for Macintosh

14 GLUT OpenGL Utility Library (GLUT) –Provides functionality common to all window systems Open a window Get input from mouse and keyboard Menus Event-driven –Code is portable but GLUT lacks the functionality of a good toolkit for a specific platform Slide bars

15 Software Organization GLUT GLU GL GLX, AGL or WGL X, Win32, Mac O/S software and/or hardware application program OpenGL Motif widget or similar

16 OpenGL Architecture Immediate Mode Display List Polynomial Evaluator Per Vertex Operations & Primitive Assembly Rasterization Per Fragment Operations Texture Memor y CPU Pixel Operations Frame Buffer Geometric pipeline

17 OpenGL Functions Primitives –Points –Line Segments –Polygons Attributes Transformations –Viewing –Modeling Control Input (GLUT)

18 OpenGL State OpenGL is a state machine OpenGL functions are of two types –Primitive generating Can cause output if primitive is visible How vertices are processes and appearance of primitive are controlled by the state –State changing Transformation functions Attribute functions

19 Lack of Object Orientation OpenGL is not object oriented so that there are multiple functions for a given logical function, e.g. glVertex3f, glVertex2i, glVertex3dv,….. Underlying storage mode is the same Easy to create overloaded functions in C++ but issue is efficiency

20 OpenGL function format glVertex3f(x,y,z) belongs to GL library function name x,y,z are floats glVertex3fv(p) p is a pointer to an array

21 OpenGL #defines Most constants are defined in the include files gl.h, glu.h and glut.h –Note #include should automatically include the others –Examples –glBegin(GL_PLOYGON) –glClear(GL_COLOR_BUFFER_BIT) include files also define OpenGL data types: Glfloat, Gldouble,….

22 A Simple Program Generate a square on a solid background

23 simple.c #include void mydisplay(){ glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_POLYGON); glVertex2f(-0.5, -0.5); glVertex2f(-0.5, 0.5); glVertex2f(0.5, 0.5); glVertex2f(0.5, -0.5); glEnd(); glFlush(); } int main(int argc, char** argv){ glutCreateWindow("simple"); glutDisplayFunc(mydisplay); glutMainLoop(); }

24 Event Loop Note that the program defines a display callback function named mydisplay –Every glut program must have a display callback –The display callback is executed whenever OpenGL decides the display must be refreshed, for example when the window is opened –The main function ends with the program entering an event loop

25 Defaults simple.c is too simple Makes heavy use of state variable default values for –Viewing –Colors –Window parameters Next version will make the defaults more explicit

26 Notes on compilation See website and ftp for examples Unix/linux –Include files usually in …/include/GL –Compile with –lglut –lglu –lgl loader flags –May have to add –L flag for X libraries –Mesa implementation included with most linux distributions –Check web for latest versions of Mesa and glut

27 Compilation on Windows Visual C++ –Get glut.h, glut32.lib and glut32.dll from web –Create a console application –Add opengl32.lib, glut32.lib, glut32.lib to project settings (under link tab) Borland C similar Cygwin (linux under Windows) –Can use gcc and similar makefile to linux –Use –lopengl32 –lglu32 –lglut32 flags

Programming with OpenGL Part 2: Complete Programs

29 Objectives Refine the first program –Alter the default values –Introduce a standard program structure Simple viewing –Two-dimensional viewing as a special case of three-dimensional viewing Fundamental OpenGL primitives Attributes

30 Program Structure Most OpenGL programs have a similar structure that consists of the following functions –main() : defines the callback functions opens one or more windows with the required properties enters event loop (last executable statement) –init() : sets the state variables viewing Attributes –callbacks Display function Input and window functions

31 Simple.c revisited In this version, we will see the same output but have defined all the relevant state values through function calls with the default values In particular, we set –Colors –Viewing conditions –Window properties

32 main.c #include int main(int argc, char** argv) { glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay); init(); glutMainLoop(); } includes gl.h define window properties set OpenGL state enter event loop display callback

33 GLUT functions glutInit allows application to get command line arguments and initializes system gluInitDisplayMode requests properties of the window (the rendering context) –RGB color –Single buffering –Properties logically ORed together glutWindowSize in pixels glutWindowPosition from top-left corner of display glutCreateWindow create window with title “simple” glutDisplayFunc display callback glutMainLoop enter infinite event loop

34 init.c void init() { glClearColor (0.0, 0.0, 0.0, 1.0); glColor3f(1.0, 1.0, 1.0); glMatrixMode (GL_PROJECTION); glLoadIdentity (); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); } black clear color opaque window fill with white viewing volume

35 Coordinate Systems The units of in glVertex are determined by the application and are called world or problem coordinates The viewing specifications are also in world coordinates and it is the size of the viewing volume that determines what will appear in the image Internally, OpenGL will convert to camera coordinates and later to screen coordinates

36 OpenGL Camera OpenGL places a camera at the origin pointing in the negative z direction The default viewing volume is a box centered at the origin with a side of length 2

37 Orthographic Viewing z=0 In the default orthographic view, points are projected forward along the z axis onto the plane z=0

38 Transformations and Viewing In OpenGL, the projection is carried out by a projection matrix (transformation) There is only one set of transformation functions so we must set the matrix mode first glMatrixMode (GL_PROJECTION) Transformation functions are incremental so we start with an identity matrix and alter it with a projection matrix that gives the view volume glLoadIdentity (); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);

39 Two- and three-dimensional viewing In glOrtho(left, right, bottom, top, near, far) the near and far distances are measured from the camera Two-dimensional vertex commands place all vertices in the plane z=0 If the application is in two dimensions, we can use the function gluOrtho2D(left, right,bottom,top) In two dimensions, the view or clipping volume becomes a clipping window

40 mydisplay.c void mydisplay() { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_POLYGON); glVertex2f(-0.5, -0.5); glVertex2f(-0.5, 0.5); glVertex2f(0.5, 0.5); glVertex2f(0.5, -0.5); glEnd(); glFlush(); }

41 OpenGL Primitives GL_QUAD_STRIP GL_POLYGON GL_TRIANGLE_STRIP GL_TRIANGLE_FAN GL_POINTS GL_LINES GL_LINE_LOOP GL_LINE_STRIP GL_TRIANGLES

42 Polygon Issues OpenGL will only display polygons correctly that are –Simple: edges cannot cross –Convex: All points on line segment between two points in a polygon are also in the polygon –Flat: all vertices are in the same plane User program must check if above true Triangles satisfy all conditions nonsimple polygon nonconvex polygon

43 Attributes Attributes are part of the OpenGL and determine the appearance of objects –Color (points, lines, polygons) –Size and width (points, lines) –Stipple pattern (lines, polygons) –Polygon mode Display as filled: solid color or stipple pattern Display edges

44 RGB color Each color component stored separately in the frame buffer Usually 8 bits per component in buffer Note in glColor3f the color values range from 0.0 (none) to 1.0 (all), while in glColor3ub the values range from 0 to 255

45 Indexed Color Colors are indices into tables of RGB values Requires less memory –indices usually 8 bits –not as important now Memory inexpensive Need more colors for shading

46 Color and State The color as set by glColor becomes part of the state and will be used until changed –Colors and other attributes are not part of the object but are assigned when the object is rendered We can create conceptual vertex colors by code such as glColor glVertex glColor glVertex

47 Smooth Color Default is smooth shading –OpenGL interpolates vertex colors across visible polygons Alternative is flat shading –Color of first vertex determines fill color glShadeModel (GL_SMOOTH) or GL_FLAT

48 Viewports Do not have use the entire window for the image: glViewport(x,y,w,h) Values in pixels (screen coordinates)